RF shields are used on printed-circuit (PC) boards to prevent electromagnetic interference (EMI) both by and of devices and links on the PC boards. State-of-the art circuit-assembly techniques use automation, whereby components are automatically placed by machine on PC boards and are then automatically soldered. Conventional drilled-through PC boards have component mounting holes extending through the board for mounting leads of the components. Automatic assembly techniques for these boards rely on robotics wherein finger-like devices pick up the components--including RF shields--and insert their leads into the appropriate holes.
Recently, surface-mount technology has been replacing drilled-through PC boards. In surface-mount technology, component mounting pads that are formed on the surface of the PC board replace mounting holes. During assembly, these pads are covered with solder paste, components are automatically placed on the pads by means of vacuum placement machines, and the assemblage is heated to melt the solder paste and thus attach the components to the PC board. The surface-mount technology is less expensive to practice. Inter alia, the elimination of robotic automatic-assembly machines and substitution therefor of vacuum-placement machines results in significant savings during circuit assembly.
Unfortunately, metallic RF shields have been considered to be too heavy for vacuum placement during surface-mount circuit assembly, as disclosed in U.S. Pat. No. 5,235,131. Consequently, it has been thought necessary to either place the metallic RF shields by hand or by robotic machines, or to replace metallic RF shields with other types of RF shields that are suited for vacuum placement as taught, for example, in U.S. Pat. No. 5,235,131. Hand placement of RF shields is expensive and also violates the concept of automated assembly. Robotic placement is expensive because it requires the addition of a robotic-placement machine to the conventional vacuum-placement machines in a surface-mount automated assembly line. And substitution of esoteric new types of RF shields for conventional metallic RF shields prevents use of these time-tested, effective, and inexpensive components in automated vacuum-placement surface-mount assembly.
Many circuit packs make use of a large mix of surface mount components with widely yawing thermal masses. During the reflow soldering operation, it is important to melt the solder at all joints and yet not damage any component due to excessive heating. To do this, it is sometimes necessary to place thermal shields over selected components to prevent thermal damage. Hand or robotic assembly of thermal shields is expensive and violates the concept of automated assembly.